I am one of Southwest's "test planes" for in-flight wi-fi. Not bad. A little slow and I got kicked off once, but probably no different that being in an average coffee shop.
During test marketing it is free, but it will be a paid item once it rolls out. This is a nice feature for quick flights to get a few emails out before you get to your meeting, but where it really is needed is for long-haul flights when you need to kill 10-12 hours.